Silicide Schottky-barrier detectors are among the most promising infrared sensors for large focal plane array applications due to their advantages of uniformity, reliability, and low cost. State-of-the-art silicide PtSi focal plane arrays are used for imaging in the 3-5 .mu.m medium wavelength infrared (MWIR) region. 640.times.480 and 1024.times.1024 element PtSi imaging arrays have been demonstrated:
Infrared Technology XVII, edited by I. J. Spiro, B. F. Andresen and M. Scholl, Proc. SPIE, Vol. 1540, pp. 285-296 (1991) by D. J. Sauer, F. V. Shallcross, F. L. Hsueh, G. M. Meray, P. A. Levine, H. R. Gilmartin, T. S. Villani, B. J. Esposito, and J. R. Tower;
Infrared Technology XVII, edited by B. F. Andresen, M. Scholl and I. J. Spiro, Proc. SPIE, Vol. 1540, pp. 297-302 (1991) by J. L. Gates, W. G. Connelly, T. D. Franklin, R. E. Mills, F. W. Price and T. Y. Wittwer;
Infrared Technology XVII, edited by B. F. Andresen, M. Scholl and I. J. Spiro, Proc. SPIE, Vol. 1540, pp. 303-311 (1991) by D. L. Clark, J. R. Berry, G. L. Compagna, M. A. Cosgrove, G. G. Furman, J. R. Heydweiller, H. Honickman, R. A. Rehberg, P. H. Solie, and E. T. Nelson;
"High Performance 1040.times.1040 Element PtSi Schottky-Barrier Image Sensor", Infrared Technology XVIII, edited by B. F. Andresen and F. D. Shepererd, Proc. SPIE, Vol. 1762, (1992) by M. Kimata, N. Yutani and S. N. Tsubouchi.
The PtSi spectral response follows the Fowler dependence, and its quantum efficiency (QE) is given by ##EQU1## where C.sub.1 is the emission coefficient, hv and .lambda. are the energy and the wavelength of the incident photon, respectively, q.phi..sub.B is the Schottky barrier height, and .lambda..sub.c is the cutoff wavelength, given by ##EQU2##
The Schottky barrier height of the PtSi detector is .about.0.22 eV, corresponding to a cutoff wavelength of .about.5.6 .mu.m. Due to the Fowler dependence, the QE of the PtSi detector in the 3-5 .mu.m MWIR regime is relatively low.
There is a great interest in extending the PtSi cutoff wavelength for long wavelength infrared (LWIR) operation in the 8-14 .mu.m regime and for improved MWIR performance. The Schottky barrier height is determined by the combined effects of the image-force effect and the electric field of the depletion region. Consequently, the effective PtSi Schottky barrier height can be reduced by introducing a thin p-type doped layer at the silicide/Si interface (S. M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981, Chap. 5). Due to the enhanced electric field of the doping spike, a potential spike near the PtSi/Si interface was formed, allowing photo-excited holes to tunnel into the substrate, resulting in a lower effective potential barrier. Previously, very shallow ion implants at the metal-silicon interface, first demonstrated by Shannon (J. M. Shannon, Appl. Phys. Lett., 24, 369, 1974), have been employed by Pellegrini et al. and Wei et al. to extend the PtSi cutoff wavelength (P. Pellegrini, M. Week, and C. E. Ludington, Proc. SPIE, Vol. 311, Mosaic Focal Plane Methodologies II, edited by Chan and Hall, pp. 24-29, 1981; and C. Y. Wei, W. Tantraporn, W. Katz, and G. Smith, Thin Solid Films, 93, 407, 1982). More recently, molecular beam epitaxy (MBE) was used to grow the thin doping spikes to reduce the Schottky barriers of Ti/Si (W. C. Ballamy and Y. Ota, Appl. Phys. Lett., 39, 629, 1981) and CoSi/Si.sub.2 (R. W. Fathauer, T. L. Lin, P. J. Grunthaner, J. Maserjian, and P. O. Anderson, Proc. SPIE, Vol. 877, Innovative Science and Technology, edited by C. Kukkonen, pp. 2-7, 1988. However, the additional tunneling process required for the collection of the photo-excited carriers reduces the detector response. Furthermore, due to the limited abruptness of the implanted doping spike profile, the electric field near the doping spike was increased drastically, resulting in a significantly increased contribution of tunneling current to the detector dark current.
The following U.S. Patents are related to the invention:
U.S. Pat. No. 4,533,933 Pellegrini et al PA1 U.S. Pat. No. 4,374,012 Adlerstein PA1 U.S. Pat. No. 4,544,939 Kosonocky et al PA1 U.S. Pat. No. 4,559,091 Allen et al PA1 U.S. Pat. No. 4,806,502 Jorke et al PA1 U.S. Pat. No. 4,908,686 Maserjian PA1 U.S. Pat. No. 4,929,064 Schubert PA1 U.S. Pat. No. 5,023,685 Bethea et al PA1 U.S. Pat. No. 5,132,763 Maserjian PA1 U.S. Pat. No. 5,161,235 Shur et al PA1 U.S. Pat. No. 5,163,179 Pellegrini PA1 U.S. Pat. No. 5,241,197 Murakami et al PA1 U.S. Pat. No. 5,285,098 Borrello
Of the foregoing patents, the following are the most relevant:
U.S. Pat. No. 4,908,686 to Maserjian is directed to a PtSi infrared detector capable of operation in the long-wavelength region. Referring to FIGS. 2 and 3, you will note that the detector is formed with a Schottky-barrier structure wherein two silicon layers 12 sandwich a metal silicide layer 10. Of particular relevance, is the disclosure that a dopant spike 22 is introduced adjacent the silicon/silicide interface 16. The detector structure is formed utilizing a molecular beam epitaxy process with P+ doping. However, this design relies on tunneling of holes into the silicon which is a direct opposite of the teachings of the present invention.
U.S. Pat. No. 4,544,939 to to Kosonocky et al is directed to a long-wavelength infrared detector in the form of a Schottky-barrier diode. As shown in FIG. 2, the Schottky-barrier detector 10' includes a shallow ion implanted P+ layer 15 at the PtSi/Si interface, for lowering the barrier for the photoemission of carriers and thereby extending the long-wavelength spectra response of the detector. However, this disclosure also relies on tunneling (see column 4, line 64).
U.S. Pat. No. 5,132,763 to Maserjian is directed to an indium arsenide long-wave-infrared detector. Although not directed to PtSi devices, the reference discloses device fabrication utilizing MBE and a structure wherein the interface between adjacent layers 14' and 16' is doped utilizing "spike" doping.
U.S. Pat. No. 5,285,098 to Borrello is directed to a photoemission detector having increased quantum efficiency in the infrared range. The device is constructed utilizing a silicide film, which may be a platinum silicide, the device having a grooved structure formed by etching for increasing the absorption in the long-wavelengths. Further efficiency increase can be obtained by increasing the free-carrier absorption by higher P+ doping.
U.S. Pat. No. 4,929,064 to Schubert is directed to an optical modulator. Although not directed to a PtSi device, this opto-electronic device employs molecular-beam epitaxy for formation thereof and selective doping to form doping spikes.